This invention relates to a curing composition and method for curing fresh concrete and more particularly, to a water dispersible curing composition and a method for properly curing fresh concrete by providing a membrane layer over the exposed surfaces of the concrete to retain the water of hydration therein.
Fresh concrete used in the concrete industry for constructing and repairing pavements, highways, buildings, and the like are typically coated with a curing agent to retain the water of hydration within the concrete. In order for the fresh concrete to properly cure, sufficient water must be present in the concrete mix to hydrate the silicate and aluminate compounds that make up the cement. Either a deficiency or an excess of water in the concrete during the curing process will result in the concrete not achieving its necessary or desired strength and may even result in shrinkage, cracking, or the concrete having relatively low abrasion resistance. Therefore, formulations for making concrete normally includes only the water required for hydration thereby requiring the rate of water loss during the curing process to be kept at a minimum. Various materials, methods, and procedures for curing concrete have been developed but the principles involved are the same; to insure the maintenance of a satisfactory moisture content and temperature so that desired properties may develop. It has been found that a 27 day period of water retention is sufficient for proper curing of the concrete.
During the initial stages of curing, water evaporates at the exposed surfaces of the concrete. The two systems typically used for maintaining a satisfactory moisture content in fresh concrete are 1) the continuous or frequent application of water through ponding, sprays, steams, or saturated cover materials such as burlap, dirt, sand, straw, and the like; and 2) the use of materials such as sheets of plastic, or by the application of a membrane-forming curing composition over the exposed surfaces of the fresh concrete to reduce the rate of water loss from the concrete by evaporation.
There are several advantages in using liquid membrane-forming curing compositions that make their use preferable over other curing methods. For example, the moisture within the concrete is sealed in so there is less risk of excess drying due to failure to keep the surface wet. In addition, liquid membrane-forming curing compositions are typically easy to apply, are relatively inexpensive, and require little or no cleanup.
Accordingly, to be effective, liquid membrane-forming concrete curing compositions must operate such that the moisture content is maintained within the concrete during the curing period to allow the desired levels of concrete properties to develop and to reduce the risk of shrinkage, cracking, dusting, scaling, and crazing of the concrete. This can be chemically challenging due to the relatively high pH levels of the water of hydration that can typically range between a pH 11 to a pH 14. To ensure such effectiveness, the concrete industry has developed a standard specification known as the American Society for Testing and Material's Standard Specification For Liquid Membrane-Forming Compounds For Curing Concrete (ASTM C-309). According to ASTM C-309, an effective liquid membrane-forming curing composition should create a hydrophobic membrane over the surface of the concrete such that less than 0.55 kilograms of water escapes through a square meter of the concrete during a 72 hour period.
Various liquid member-forming concrete curing compositions have been developed which are effective for maintaining a satisfactory moisture content in freshly poured concrete. Such compositions include refined linseed oil diluted in mineral sprits (petroleum distillates), acrylic resins dissolved in mineral sprits, and hydrocarbon resins or slack waxes dissolved in mineral sprits.
Curing compositions containing mineral sprits, however, have been criticized since their use may result in the releasing of volatile organic compounds (VOC's) into the atmosphere, the production of objectionable orders, and the causing of skin or respiratory irritation to humans exposed to such curing compositions. The use of such curing compositions has also been criticized because of their general high degree of flammability and the potential for environmental problems caused by spillage, run off and overspray during application, their tendency to stain the concrete surface, and their use of non-renewable raw materials. To reduce the overall amount of VOC's being released into the atmosphere, such compositions are often emulsified into water. However, such emulsified compositions generally require the use of volatile distillates, alcohols, or other environmentally unfriendly solvents (VOC's), and can be flammable and a cause of skin and respiratory irritation to humans.
Compositions containing slack waxes, while relatively inexpensive, tend to lack consistency which may result in emulsion or dispersion instability as well as finished product and performance inconsistency.
Other compositions developed for use in the concrete industry operate to form heavy polymeric coatings over the surface of the concrete. Unfortunately, while such coatings are successful in retaining water of hydration within the curing concrete, they often cause adhesion problems for floor finishers after the 27 day curing period has passed. Accordingly, the removal of such polymeric coatings from the surface of the cured concrete is often necessary before a flooring adhesive or a finish can be applied. Further, pinholes in the polymeric coatings, such as an acrylic resin-based coating, can occur and cause localized curing performance failures.
Consequently, a need exists for a liquid curing composition for concrete which is effective for forming a hydrophobic membrane over the surface of the concrete and provides product and performance consistency, which is non-toxic, non-staining, not flammable, stable, ecologically acceptable, relatively inexpensive, and easily applied.